Unlike some 3-year-olds, Derek Wong of West Los Angeles rarely fussed in the morning about having to go to day care. Part of the reason, acknowledge his parents, Debbie and Rholan Wong, was Derek's beloved ritual.

First, he would switch on the television and settle down to watch the "Teenage Mutant Ninja Turtles" cartoon over breakfast. Then he would pull on one of his six Ninja Turtle T-shirts, put on Turtle socks and shorts, and grab a stuffed Turtle toy or a Turtle sword. Slicing it through the air, he would climb gleefully into the car, exclaiming, "Turtles fight with honor!"

Like many other toddlers, Derek had fallen under the spell of the pizza-loving, sword-carrying reptiles that have enjoyed the limelight ever since last spring's blockbuster movie of the same name.

Until his day-care provider lowered the boom with a new decree.

"We're saying goodby to the Turtles," she recently announced to her 12 charges. "No more Turtle T-shirts or toys; no more words like 'cowabunga (a Turtle favorite).' Turtles are fine at home, but not at Mary's."

In an interview, Mary Alvidrez, a West Los Angeles child-care provider, said: "My decision was based on the children's actions, the way they were playing. They would play Turtles and imitate them. They got a little carried away. The older kids would do karate on the younger ones. They were very much into fantasy play."

Some other Southern California day-care centers and preschools also are waging war against the reptiles, concerned that Turtle play is sparking violence and aggression.

Some day-care centers and schools have banned Turtle wear and toys. Others forbid Turtle weapons, but allow children to wear shirts and other gear. Some ration Turtle power, allowing children to show off their favorite Turtle toys one or two days a week at "share" time. And anti-war activist Jerry Rubin, director of the Los Angeles Alliance for Survival, is offering Teddy bears to kids who turn in their Turtles ((213) 399-1000).

To complicate matters, parents, child-care providers and psychologists don't see eye-to-eye on the effects of the swashbuckling teen-age mutants. Some think nothing short of a Turtle ban is enough; others see little difference between the animated Turtles and the cowboy-and-Indian games of yesteryear.

On one point most would agree: Turtles are more pervasive than the Roy Rogers and Dale Evans gun-and-holster sets of the '50s and '60s. Since the Turtles first debuted in the comics in 1984, merchandising has taken off. There are Turtle action figures and accessories, computer games, videos, stuffed animals and bubble gum.

"The toy trade projects sales of more than $200 million this year," said Diane Teigiser, director of marketing for Playmates Toys Inc. in La Mirada, the licensee for Turtle action figures. "Last year, we sold $110 million just in Turtle action figures and action figure accessories."

Prime fans are boys ages 3 to 8, but the "heroes in a half shell" also claim little girls and older children as fans.

The story is an enticing one to young minds. Four pet turtles dropped into a sewer are covered with radioactive goo and transformed into teens with unlikely names: Leonardo, Donatello, Michaelangelo and Raphael. They're taught the ways of the Ninja warrior by Splinter, "the biggest rat ever to face a trap." Their enemies are a collection of mutant characters called the Foot Clan--Bebop, Shredder and Rocksteady among them.

It's overkill, claimed Dr. Carole Lieberman, a Beverly Hills psychiatrist and UCLA assistant clinical professor of psychiatry who supports a Turtle ban. "Kids become more violent in reaction to the Turtles," she claims. "It gets other kids more riled up and play turns more aggressive."

Other behavior experts propose a middle ground, reasoning that a ban might make the reptiles even more irresistible to children. The swashbuckling Turtles aren't much different than the cowboys or soldiers of years past, said Gary Emery, a Los Angeles psychologist.

But Lieberman points out what she sees as a crucial difference. "Ten or 20 years ago, parents were around more to give kids values, to comment about the violence (children encountered)," she said. "These days, many kids are playing with these toys in a vacuum. And kids are often angry to begin with because they are left alone."

"A toy does not cause a child to be violent," countered Teigiser of Playmates Toys. "Children are going to act out their aggression whether with their toys or their fingers."

The Turtles may be taking more than their fair share of blame for rambunctious behavior, agreed Emery. "Certain developmental behaviors will occur no matter what," he said. "These Turtle fans may be simply using a cultural artifact to act out a developmental task--in this case, to run around and act a little crazy." It's akin, Emery contends, to the teen rebellion that comes later.

Other experts--and some parents--also worry that young Turtle fans may have difficulty distinguishing reality and fantasy.